Nuclear Magnetic Resonance may provide a new and powerful method for use in cancer. The method appears to be non-disruptive permitting direct spectroscopic evaluation of tumor tissue providing the possibility of rapid quantitative chemical analysis of surgical, autopsy, and cytological specimens. The first nucleus studied by this method is the proton of water. Spin echo relaxation times, T1 and T2, have been found to be prolonged in the cancer tissues thus far studied. T1 and T2 measurements of water protons in 2 malignant tumors (Novikoff Hepatoma and Walker Sarcoma) derived from the rat produced T1 values that were distinctly greater (Walker Sarcoma .736 secs., Novikoff Hepatoma .826 secs.) than all of the normal tissues studied (muscle .531 secs., liver .293 secs., stomach .270 secs., small intestine .257 secs., kidney .480 secs., brain .595 secs.).